1. Field of the Invention
The present invention relates to gas turbine engines and more specifically to the construction of turbine disk and blade assemblies for use in high temperature gas turbine engines.
2. Description of the Prior Art
The construction of disk and blade assemblies for gas turbine engines has always required precise engineering to ensure structural integrity of individual components during engine operation. Significant efforts are being made to reduce the high centrifugal and thermal stresses often occurring in individual engine components and to reduce the performance losses resulting from the leakage of hot gases through the turbine blade attachment region to a lower pressure stage. Similarly, efforts are being made to use cooling air more efficiently.
Turbine rotor assemblies in gas turbine engines consist of one or more stages of disk and blade assemblies wherein individual blades are attached to the rim of a turbine disk and extend in a radially outward direction. During rotation of the disk, tensile stresses occur in the blades as a result of centrifugal force. The magnitude of these stresses is highest in the blade root region of the turbine disk and complex attachment shapes have been adopted in order to reduce these stresses. A typical attachment means is the fir tree type connector shown in U.S. Pat. No. 3,666,376 to Damlis. The turbine disks which are exposed to the hottest gases are sometimes formed with cooling air plenum chambers in the base of the fir tree slot in the disk as is also shown in Damlis. Cooling air enters the chamber by any of a number of distribution means and leaves the plenum chamber traveling radially through the root portion of the blades and ultimately exiting through cooling holes on the airfoil section of the turbine blades.
In order to accommodate tangential thermal growth of the turbine blades and turbine blade platforms, modern engines are constructed with tangential clearance between the blade platforms of adjacent turbine blades. The clearance accommodates thermal cycling in the engine; however, the blades tend to vibrate unless damped. Damlis shows the use of a turbine blade damper in a substantially axial cavity defined by the turbine blade roots, the turbine disk and adjacent turbine blade platforms. The damping member can be restrained within the platform by various means including a tab and slot arrangement.
Gas leakage in the axial direction from the high to the low pressure side of the disk and blade assembly is not uncommon. The leakage occurs through the turbine root cavity, the turbine blade damper cavity, and the interface between the turbine blade root and the disk at the fir tree connector. Various constructions have been divised to prevent this leakage. For example, Damlis shows a typical construction in which annular sideplates are riveted to the turbine disk. Pratt et al show annular sideplates attached to the opposing faces of the turbine disk and secured by interlocking dogs on the disk and sideplates in U.S. Pat. No. 3,096,074. A similar construction in which the sideplates are attached to the disk with radially aligned pins is disclosed in U.S. Pat. No. 3,455,537. In U.S. Pat. No. 3,295,825, Hall teaches the use of sideplates attached to the turbine disk with a bayonet type connector, a separate sideplate being secured to each face of the disk with L-shaped lugs which extend from the turbine blade root region.
Each of the constructions described above is useful, however, as operating temperatures of gas turbines continue to increase, the thermal and centrifugal forces have a greater tendency to warp the sideplates thereby interrupting the surface contact between the sideplates and the turbine disks. The amount of gas leakage across the turbine disk is directly related to the extent to which the surface contact is disrupted. In addition, annular sideplates experience stress concentrations due to thermal and centrifugal loads and fracture of the sideplates or of the sideplate attaching members can result.
The segmentation of sideplates with provisions for adequate clearance between the segments can reduce the sideplate deformation resulting from centrifugal and thermal forces, however, warping and gas leakage are still excessive.